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Samples of CoFe 2 O 4 /MgO(x) (where x is a series concentration of the MgO, x = 0.00, 0.02, 0.04, 0.06, 0.08) are successfully synthesized by a sol-gel auto-combustion technique using spent LIBs as the raw materials. The effects of MgO concentration on the resultant phase purity, particle size, lattice parameter, micro-structure, magnetic and magnetostrictive properties are investigated. There are significant changes with saturation magnetization, magnetostrictive coefficient and strain derivative when mixing MgO powders with different masses. In contrast, the best magnetic and magnetostrictive properties of the nanoparticles occurs when the mass ratio of CoFe 2 O 4 :MgO = 1:0.02. The saturation magnetization, magnetostrictive coefficient and the strain derivative of the CoFe 2 O 4 /MgO(0.02) nanocomposites are 77.35 emu/g, −118.6 ppm and −2.16 × 10−9 A−1m, respectively. The CoFe 2 O 4 /MgO(x) nanocomposites exhibit huge potential for future use in the fields of non-contact sensors and actuators with low-magnetic fields. • Few studies focused on the magnetostrictive property CoFe 2 O 4 nanoparticles obtained from mixtures of different MgO masses. • The maximum magnetostrictive strain derivative is -2.16×10-9A-1m, for the CoFe 2 O 4 /MgO(x=0.02) nanocomposite. • The CoFe 2 O 4 /MgO(x) nanocomposites exhibit huge potential at low magnetic fields for the automotive sensors field. [ABSTRACT FROM AUTHOR]

Recovering spent Li-ion batteries is beneficial to the economy and environment. Therefore, this study synthesized nanoparticles of cobalt ferrite doped with different rare earth ions (Nd, Ce, and Pr) by a sol-gel auto-combustion method using spent Li-ion batteries. The effect of the different doping elements on grain sizes, structure, magnetic and magnetostrictive properties, and strain derivative were confirmed by X-ray diffraction, scanning election microscopy, vibrating sample magnetometer, and a magnetostrictive coefficient measuring system. Substitution of a small amount of Fe 3+ with RE 3+ in CoRE x Fe 2-x O 4 (x = 0.025, 0.05, and 0.1) had a large effect on magnetostrictive properties and strain derivative, which was improved compared with pure cobalt ferrite at low magnetic field. The maximum strain derivative (dλ/dH = −1.49 × 10 −9  A −1  m at 18 kA m −1 ) was obtained for Nd, x = 0.05. Changes in the magnetostriction coefficients and strain derivatives were correlated with changes in cation distribution, microstructure, and magnetic anisotropy, which depended strongly on RE 3+ substitution and distribution in the spinel structure. [ABSTRACT FROM AUTHOR]